Neuroblastoma x glioma NG108-15 hybrid cells possess alpha-adrenergic receptors that, when activated by an alpha-adrenergic agonist such as norepinephrine, mediate inhibition of adenylate cyclase. Affinity labels are being designed to determine the macromolecule(s) constitution the NG108-15 alpha-receptor. A number of chemically reactive derivatives of alpha-adrenergic agonist and antagonist compounds are being synthesized and tested for their ability to bind specifically to and react covalently with alpha-receptors. In addition, novel alpha-receptor ligands that can be iodinated to high specific radioactivity are being synthesized and tested for interaction with NG108-15 and brain alpha-receptors.